How to wait for bpy.ops.wm.alembic_import to finish?
bpy.ops.wm.alembic_import operator provides a property called as_background_job which defaults to True. In that case the alembic import will happen asynchronously.

You can override the default behavior by passing a False value, in which case the import happens synchronously (blocking Blender)

bpy.ops.wm.alembic_import(filepath='....abc', as_background_job=False)